Add basic weather information to screensaver

# Weather Widget for cinnamon-screensaver Adds a new type of floating widget (similar to Clock and AlbumArt) that displays a weather icon alongside the current weather conditions. Utilizes OpenWeatherMap for weather data and GeoJS.io for location data. Models and interfaces are designed to be mostly compatible with the Weather applet in [cinnamon-spices-applets/weather@mockturtl](https://github.com/linuxmint/cinnamon-spices-applets) so that additional Weather and Location providers can be easily added. The OpenWeatherMap API key from the cinnamon-spices-applets has been reused to enable out-of-the-box operation. ## Dependencies App will crash until the dconf schema is updated. PR with the new schema in cinnamon-desktop is here: https://github.com/linuxmint/cinnamon-desktop/pull/246 PR with updated cinnamon-settings screensaver module here: https://github.com/linuxmint/cinnamon/pull/12516 In the meantime: 1. download [the new schema file from the above PR](https://github.com/queenkjuul/cinnamon-desktop/blob/screensaver-weather/schemas/org.cinnamon.desktop.screensaver.gschema.xml.in) 2. Backup `/usr/share/glib-2.0/schemas/org.cinnamon.desktop.screensaver.gschema.xml` 3. Move the downloaded `org.cinnamon.desktop.screensaver.gschema.xml.in` file to `/usr/share/glib-2.0/schemas/org.cinnamon.desktop.screensaver.gschema.xml` (rename it to remove the `.in` at the end) 4. run `sudo glib-compile-schemas /usr/share/glib-2.0/schemas/ ` 5. use `dconf-editor` or another means to set the relevant settings in `org.cinnamon.desktop.screensaver`